Semiconductor devices and liquid crystal display devices are produced by means of the so-called photolithography technique in which a pattern formed on a mask is transferred onto a photosensitive substrate. The exposure apparatus, which is used in the photolithography step, includes a mask stage for supporting the mask and a substrate stage for supporting the substrate. The pattern on the mask is transferred onto the substrate via a projection optical system while successively moving the mask stage and the substrate stage. In recent years, it is demanded to realize the higher resolution of the projection optical system in order to respond to the further advance of the higher integration of the device pattern. As the exposure wavelength to be used is shorter, the resolution of the projection optical system becomes higher. As the numerical aperture of the projection optical system is larger, the resolution of the projection optical system becomes higher. Therefore, the exposure wavelength, which is used for the exposure apparatus, is shortened year by year, and the numerical aperture of the projection optical system is increased as well. The exposure wavelength, which is dominantly used at present, is 248 nm of the KrF excimer laser. However, the exposure wavelength of 193 nm of the ArF excimer laser, which is shorter than the above, is also practically used in some situations. When the exposure is performed, the depth of focus (DOF) is also important in the same manner as the resolution. The resolution R and the depth of focus δ are represented by the following expressions respectively.R=k1·λ/NA  (1)δ=±k2·λ/NA2  (2)
In the expressions, λ represents the exposure wavelength, NA represents the numerical aperture of the projection optical system, and k1 and k2 represent the process coefficients. According to the expressions (1) and (2), the following fact is appreciated. That is, when the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ is narrowed.
If the depth of focus δ is too narrowed, it is difficult to match the substrate surface with respect to the image plane of the projection optical system. It is feared that the focus margin is insufficient during the exposure operation. Accordingly, the liquid immersion method has been suggested, which is disclosed, for example, in International Publication No. 99/49504 as a method for substantially shortening the exposure wavelength and widening the depth of focus. In this liquid immersion method, the space between the lower surface of the projection optical system and the substrate surface is filled with a liquid such as water or any organic solvent to form a liquid immersion area so that the resolution is improved and the depth of focus is magnified about n times by utilizing the fact that the wavelength of the exposure light beam in the liquid is 1/n as compared with that in the air (n represents the refractive index of the liquid, which is about 1.2 to 1.6 in ordinary cases).
In the case of the liquid immersion exposure apparatus as disclosed in International Publication No. 99/49504 described above, the liquid in the liquid immersion area formed on the substrate makes contact with the optical element which is arranged most closely to the image plane among a plurality of elements (optical elements) for constructing the projection optical system. In such a situation, the following possibility arises. That is, if the liquid in the liquid immersion area is mixed with any impurity or the like generated, for example, from the surface of the substrate, and the liquid in the liquid immersion area is contaminated therewith, then the optical element, which is arranged most closely to the image plane, may be polluted with the contaminated liquid in the liquid immersion area. If the optical element is polluted, any inconvenience arises, for example, such that the light transmittance of the optical element is lowered and/or any distribution appears in the light transmittance. As a result, there is such a possibility that the exposure accuracy and the measurement accuracy, which are obtained via the projection optical system, are deteriorated.
A scanning type exposure apparatus, which exposes the substrate with the pattern formed on the mask while synchronously moving the mask and the substrate in the scanning direction, is also disclosed in International Publication No. 99/49504 described above. In the case of the scanning type exposure apparatus, it is required to realize the high speed for the scanning velocity (scanning speed) in order to improve, for example, the productivity of the device. However, if the high scanning velocity is realized, the following possibility arises. That is, it is difficult to maintain the liquid immersion area to have a desired size.